Many on-vehicle fuel cell systems include a humidifier for humidifying at least one of gas to be supplied to an oxygen electrode (i.e. a cathode electrode) and gas to be supplied to a hydrogen electrode (i.e. an anode electrode), in order to accelerate a power generation reaction of the fuel cell. FIG. 1 shows an example of a portion of a fuel cell system relevant to a fuel cell and humidifiers, in which arrows indicate the flowing directions of the gas. The fuel cell system shown in FIG. 1 includes a fuel cell 10, an anode-side humidifier 12 which humidifies hydrogen to be supplied to an anode electrode of the fuel cell 10, and a cathode-side humidifier 14 which humidifies air to be supplied to a cathode electrode of the fuel cell 10. The anode-side humidifier 12 and the cathode-side humidifier 14 humidify the gas to be supplied, via a humidification membrane provided therein.
The anode-side humidifier 12 is connected to the fuel cell 10 through anode-side gas supply piping 16. Thus, hydrogen is humidified in the anode-side humidifier 12 and then supplied to the anode electrode of the fuel cell 10 through the anode-side gas supply piping 16.
Because water is generated as a result of a chemical reaction in the cathode electrode of the fuel cell 10, discharge gas from the cathode side of the fuel cell 10 contains a great amount of water vapor. The cathode-side humidifier 14 utilizes the water content contained in the discharge gas from the cathode side to humidify the air to be supplied. The cathode-side humidifier 14 is connected to the fuel cell 10 through cathode-side gas supply piping 18 and cathode-side gas discharge piping 20. The cathode-side humidifier 14 draws the discharge gas from the cathode electrode of the fuel cell 10 via the cathode-side gas discharge piping 20 and transfers, via the humidification membrane, the water content contained in the discharge gas from the cathode electrode to the air to be supplied, thereby humidifying the air to be supplied to the cathode electrode. The air to be supplied, which is thus humidified is then supplied to the cathode electrode. The discharge gas from the cathode side of the fuel cell 10, whose water content has been transferred to the air to be supplied in the cathode-side humidifier 14, is then externally discharged through humidifier discharge piping 22.
The gas passing through the anode-side gas supply piping 16 and the cathode-side gas supply piping 18, which has been humidified by the humidifiers 12 and 14, respectively, contains a great amount of water content. Similarly, the discharge gas passing through the cathode-side gas discharge piping 20 contains water generated as a result of the reaction of the cathode electrode and therefore contains a great amount of water content. Consequently, there are cases in which condensation occurs in the piping 16, 18, and 20 interconnecting the fuel cell and the humidifiers, when the temperature of the gas within the piping decreases after the termination of the operation of the fuel cell. Condensation occurring in the piping may further cause condensed water to be stored in the fuel cell and the humidifiers.
As described above, in the fuel cell systems, it is likely that condensation will occur, especially in the piping connected to the humidifiers. If condensation occurs in the piping, condensed water is accumulated in the fuel cell and the humidifiers, and the condensed water thus accumulated freezes at a low temperature, which interferes with the start of the operation of the fuel cell. It is therefore necessary to prevent condensed water from being stored in the fuel cell and humidifier. It is particularly necessary to prevent condensed water from being stored in the fuel cell.
Further, in many cases, the humidification membrane provided in the humidifier is formed of a resin and is therefore sensitive to heat. For example, the life of some humidification membranes is shortened by approximately half with an increase in the usage temperature by approximately 10° C. It is therefore necessary to maintain the temperature around the humidifier as low as possible in order to extend the useful life of the humidification membrane.